Mineralization And Structural Setting For Dabaoshan Copper Deposit In Weixi,Yunnan

Dabaoshan copper deposit is an epithermal-mesothermal deposits in Xuelongshan high-grade metamorphic zone,the north part of Ailaoshan-Jinshajiang tectonic zone,southeastern Tibet.The metamorphism and deformation in the Xuelongshan high-grade metamorphic zone had four stages:regional deformation with amphibolite facies metamorphism at the middle and lower tectonic level;folding and ductile strike-slip shearing with green-schist facies metamorphism at the middle tectonic level;brittle to ductile deformation at the upper tectonic level during uplifting.The faults inherited the mylonite foliations are the mainly depositing ore structures.Extensive Cenozoic magmatic activities in the Xuelongshan is characteristized by small granite bodies and dykes/veins.Most of these granite intrusions had been mylonitized.LA-ICP-MS zircon U-Pb analyses showed that these granites intruded at 28,2±0.3Ma.The granite and diorite Dikes/veins are controlled by faults inherited the mylonite foliations,show that formed after ductile shear deformation or 28Ma.The Dabaoshan copper ore bodies consist of ore-bearing veins controlled by subparallel brittle-ductile fracture zones.The wall rocks are felsic mylonites.The main ore minerals are chalcocite,chalcopyrite,the gangue minerals are pyrite,quartz,calcite and siderite.The wall rock alteration is made up of silicification,carbonatization,pyritization,sericitization,and kaolinization,which is zoning from ore vein to wall rock.The geochemical characteristics of the PGE,REE,trace elements and the result of S isotope analyses indicate the ore-forming materials is derived main from the magmatism and little from the wall rock.Oxygen,hydrogen and Carbon isotope data show that the ore-forming fluid is underground water mixed with little magmatic hydrothermal.The homogenization temperature data of the fluid inclusions shows the mineralization is at a temperature span of 200 to 300? and the copper deposit is belonged to medium-low T hydrothermal type.The 40Ar/39Ar plateau age and inverse isochron age of the biotite in the ore vein are 26Ma,and the age of the mylonization granite forming the wall rocks of copper bearing veins is 28Ma.So we can speculate the mineralization is at the span of 28 to 26Ma,which is related with the uplift and ductile to brittle deformation.Based on the tectonic deformation,metallogenic characteristics of the ore deposit,origins of ore-forming matters and fluid,we speculate the mineralion of the Dabaoshan copper deposit is as follow:the ore-bearing fluids migrated upward mixed with underground water migrated downward along the brittle-ductile fracture zone,which resulted in the change of the physical and chemical condition for the ore-bearing fluid,and precipitation of copper minerals,such as chalcocite and chalcopyrite,and gangue minerals such as calcite,quartz etc.to form the ore veins controlled by brittle-ductile fracture zone.